Jeff McMahan wrote a book, published in 2009, that sought to turn on its head what we understand about morality in war which is based on the centuries old tradition of Just War Theory. In the traditional theory, the morality of war is broken down into two major areas: just reasons for going to war (jus ad bellum) and just actions in the conduct of war (jus in bello). It is accepted that there is a moral equality of combatants. Hence, even if one side was unjustified in going to war, it does not mean that the combatants on that side are war criminals unless they commit unjust actions such as deliberately killing civilians.
This can be illustrated by a good example. It is not particularly controversial to say that the Allies were the just side in the war against Nazi Germany in World War II and the Nazi side were the unjust side. According to standard theory, Nazi soldiers who were involved in permitted actions of a soldier, such as fighting other armies and not killing civilians, are not doing anything morally wrong. This means that while SS officers who were involved in killing Jews were war criminals, Rommel’s Nazi troops who stormed around North Africa but obeying the rules of war such as proportionality and not deliberately killing civilians are not morally blameworthy.
McMahan thinks this is wrong. He thinks that an unjust war should not be participated in and that soldiers are morally blameworthy if they fight for an unjust side. He would view, contrary to the standard theory, Rommel’s Nazi troops as war criminals.
A conflict can occur when a just side would kill innocent civilian without intent but as a side effect of a just action. McMahan argues that while soldiers on an unjust side should not engage in combat with a just side, innocent civilians on the unjust side are permitted to defend themselves from harm resulting from actions of the soldiers on the just side. McMahan’s case is illustrated with a very bizarre thought experiment:
Suppose that just combatants are justified in destroying a storage facility for chemical weapons, but that the destruction of the facility foreseeably creates a cloud of toxic gas that will soon engulf an area inhabited by innocent civilians. If the civilians could somehow blow the cloud of gas away, they would be permitted to do that, even if their only option was to blow it over the just combatants, who would then be killed by it.
Jeff McMahan, Killing in War, (Oxford University Press, 2009), p.47.